1. Field of the Invention
This invention relates to an active quenching circuit for single photon semiconductor avalanche photodiodes, particularily for photodies that biased with a reverse voltage higher than the breakdown voltage and operating in a triggered avalanche mode. The invention is particularly suitable for operation with a photodiode in a remote position.
2. Prior Art and Other Considerations
The operating principle of semiconductor photodiodes operating in a triggered avalanche mode (or Geiger mode avalanche diode) is well known. In particular it is known that in such devices, when biased at a reverse voltage higher than the breakdown voltage, a single charge carrier (electron or hole) generated by the absorption of a single photon can trigger an avalanche process which is self-sustaning thanks to the high intensity of the electric field. To cause the avalanche to terminate a quenching circuit is used which momentarily lowers the bias voltage below the breakdown level and then rapidly restores the initial bias so that the photodiode is immediately ready to detect the subsequent photon.
It is necessary to bear in mind that, in the technical-scientific literature, there is no universally used denomination to indicate the semiconductor avalanche devices operating as described above. They have been indicated with several denominations, such as Single Photon Avalanche Diode, SPAD; Triggered Avalanche Detector, TAD; Geiger Mode Avalanche Diode and still others. For the sake of brevity, we will hereafter use the denomination single photon avalanche diode and the acronym "SPAD" to indicate a semiconductor device operating in the triggered avalanche mode, as described above.
To operate correctly, the quenching circuit must detect the leading edge of the avalanche current coming from the photodiode and in response to it:
(a) produce a standard output pulse well synchronized with such edge; PA0 (b) apply a voltage pulse to the photodiode to lower its bias down to the breakdown voltage or below it (avalanche quenching); PA0 (c) hold the voltage across the photodiode at the quenching level for a predetermined time interval (hold-off); PA0 (d) at the end of such time interval, restore the voltage across the photodiode to the operating level with a rapid transition. The transition must be rapid so as to prevent a photon, which is arriving while the voltage has not yet reached the normal operating level but is in any case already higher than the breakdown voltage, from triggering the avalanche in the photodiode under conditions other than normal, thus determining a degradation of the performance obtained. PA0 (a) on the one hand, the quenching circuit must be sensitive to avalanche current pulses of 1 mA or less, that is, typically to signals of 50 mV or less across an input having a resistance of 50 Ohms; PA0 (b) on the other hand, the quenching pulses applied across the photodiode must have a considerable amplitude, from a few volts to several tens of volts according to the photodiode used. These pulses cannot have a perfectly ideal rectangular shape, with no oscillations or overshoots on the rapid leading and trailing edges. Even if they are well shaped, they have irregularities which, although they appear very small in percentage proportion to the amplitude of the pulse, (typically from 1 to 3%), when compared to the sensitivity of the quenching circuit they are really quite large, from tens to hundreds of mV. It is therefore necessary to prevent such inevitable irregularities in the quenching pulse from retriggering the circuit, particularly in correspondence to the trailing edge of the quenching pulse.
There are several problems which arise in obtaining the correct operation of the photodiode and of the corresponding quenching circuit. In particular it is necessary to avoid the production of more than one output pulse in response to an avalanche front, or even a persistent oscillation of the quenching circuit, which arises spontaneously or due to the arrival of an avalanche front.
Such operating irregularities are to be feared since:
A particular problem arises in relation to the requirement of operating the quenching circuit correctly, avoiding the above operating irregularities and corresponding drawbacks, even with the photodiode located in a remote position with respect to the quenching circuit and connected to it electrically by means of a transmission line, such as a coaxial cable.
This requirement arises in many situations, either to assemble within an apparatus (say, a microscope) the small photodiode detector, or to assemble the detector in a housing to keep it under controlled temperature conditions (say, a cryostat for operation under cryogenic conditions).
Mounting of a photodiode at the end of a coaxial cable drastically increases the danger of spurious retriggering of the quenching circuit. Indeed, to avoid signal reflections at the end of the cable, the latter should be terminated on a load having an impedance equal to the characteristic one of the transmission line, that is, with the cables normally in use, about 50 Ohms. But the photodiode presents a mainly capacitive impedance at the end of the cable, so that at the leading and trailing edges of the quenching pulse reflections are inevitably generated, which, when received by the quenching circuit, can cause spurious triggering.
The active quenching circuits developed so far do not permit to operate the photodiode remote from the circuit and connected to it by a coaxial cable. Furthermore, they accept quenching pulses of just a few volts, and thus permit to bias the photodiode above the breakdown voltage by only these few volts. Lastly, the adjustement of these circuits turns out to be quite critical, as concerns the operating difficulties mentioned above.
The object of the present invention is to accomplish an active quenching circuit for single photon semiconductor avalanche photodiodes, which can avoid the drawbacks mentioned above.
In particular, the object of the present invention is to accomplish an active quenching circuit which can operate without problems even if the photodiode is in a remote position.
The object of the present invention is also to accomplish a quenching circuit capable of applying quenching impulses as high as required in order to bias the photodiode well above the breakdown voltage.
Lastly, the object of the present invention is to accomplish a fast quenching circuit, well synchronized with the leading edge of the avalanche current, and being easy to adjust.